docs(readme): the consumer shape, a How to use, and a first CLAUDE.md

[jshvn]

What changes

The README takes the consumer shape used across the org and comes down from 515 lines: the lede with the provenance note, How to use (new: where the files are, what an edit and a delete become), How it works (the pipeline drawn, every migrator step dashed, the step table cut to a sentence or two each, the evidence paragraph kept), Want your own? (fork, storage, secrets, the Dropbox app and the session, prove it and seed), Operating it with the runbook trimmed to its entries, Reference (the TOML table, what sits where). The Proton session commands, the vault setup and what the reusable workflow does are linked to katoptra/lib's README (katoptra/lib#5) rather than repeated. Emoji headings go.

• A first CLAUDE.md: the must-knows (plan by default, a missing state beside history is refused, the listing floor, checkpoint last, the session is this mirror's own, flags after the double dash) and how to verify a change.
• op.env no longer names the org's vault.

Dropped from the README on purpose: the repository-layout tree (what ls says), the toolbox verb list (lib's), the paragraph on what check.yml runs (it is one job, not the reusable workflow plus a second job, as the old text said). No code changes; render.txt is untouched.

Storage and operations

None.

Test plan

• task check, task test and task lint pass (the check workflow)
• The rendered README's diagram draws and its lib links resolve once docs(readme): the manual, with every stage drawn lib#5 merges